Shrinking balloon catheter having nonlinear or hybrid compliance curve

ABSTRACT

A balloon catheter, having a non-linear compliance curve, made up of a single layered balloon, that was molded from a polymeric material to have a specific nominal diameter and then shrunk to a diameter that is less than the specific nominal diameter. When the shrunk balloon is expanded it has the characteristics of a compliant balloon until its diameter has been expanded to its original specific nominal diameter after which it follows the non compliant compliance curve that it would have had it not been shrunk.

BACKGROUND OF THE INVENTION

The present invention relates generally to balloon catheters used forangioplasty.

Angioplasty, an accepted and well known medical practice involvesinserting a balloon catheter into the blood vessel of a patient,maneuvering and steering the catheter through the patient's vessels tothe site of the lesion with the balloon in an un-inflated form. Theun-inflated balloon portion of the catheter is located within the bloodvessel such that it crosses the lesion or reduced area. Pressurizedinflation fluid is metered to the inflatable balloon through a lumenformed in the catheter to thus dilate the restricted area. The inflationfluid is generally a liquid and is applied at relatively high pressures,usually in the area of six to twelve atmospheres. As the balloon isinflated it expands and forces open the previously closed area of theblood vessel. Balloons used in angioplasty procedures such as this aregenerally fabricated by molding and have predetermined design dimensionssuch as length, wall thickness and nominal diameter. Balloon cathetersare also used in other systems of the body for example the prostate andthe urethra. Balloon catheters come in a large range of sizes and mustbe suitably dimensioned for their intended use.

The term, low pressure diameter, as used herein with reference to theballoon catheter, means the diameter of the balloon when it is inflatedto two (2) atmospheres.

The term, expanded diameter, as used herein with reference to theballoon catheter, means the diameter of the balloon when it is inflatedto six (6) to (12) atmospheres.

All angioplasty balloons have a minimum pressure at which they willburst called the minimum burst pressure. The physician is aware of theminimum burst pressure of angioplasty balloons that he or she uses andusually avoids inflating a balloon to the point where it bursts. Thephysician is also aware that each kind and size of angioplasty balloonhas its own expansion characteristics. This characteristic is usuallyexpressed as a number which is the decimal portion of a millimeter thatthe balloon will expand when one additional atmosphere of pressure isapplied. For example a 3 millimeter (low pressure diameter) balloon mayexpand 0.10 millimeters for each additional atmosphere of pressure thatis applied. In this example at 12 atmospheres of pressure the balloonwould have a diameter of 4.00 millimeters. This stretchingcharacteristic is a factor of both the wall thickness and the materialfrom which the balloon is molded. If the diameter of a balloon ismeasured during inflation, and the diameter is plotted, as onecoordinate, against the inflation pressure as the other coordinate, theresulting curve is called the compliance curve for that particularballoon. If a balloon is made of a material that results in a relativelylarge increase in diameter when the balloon is inflated to its expandeddiameter, such a balloon is said to be a High-Compliant balloon, or issaid to be a balloon with a high compliance curve.

FIG. 1A, is a graph showing a set of compliance curves for catheterballoons. In FIG. 1A the inflation pressure, measured in atmospheres, isplotted along the X-axis and the balloon diameter measured inmillimeters is plotted along the Y-axis. In this figure the compliancecurve having the greatest inclination is labeled High-Compliant. AHigh-Compliant balloon has a relatively large increase in diameter inresponse to an increase in inflation pressure. It should be noted thatballoons defined herein as High-Compliant balloons are commonly referredto in the trade as, "Compliant balloons" or balloons made from compliantplastic material.

If a balloon is made of a material that results in a relatively smallincrease in diameter when the balloon is inflated to its expandeddiameter, such a balloon is said to be a Non-Compliant balloon, aballoon made from non compliant plastic material or a balloon with a lowcompliance curve. In FIG. 1A, the compliance curve having the leastinclination is labeled Non-Compliant. A Non-Compliant balloon has arelatively small increase in diameter in response to an increase ininflation pressure. In FIG. 1A the third compliance curve is labeledIntermediate Compliant and represents a balloon having compliantcharacteristics between High and Non-Compliant balloons. It should benoted that although only three compliance curves are shown in FIG. 1A,balloons having compliant anywhere between the High-Compliant and theNon-Compliant curves are available. It should also be noted that allcompliance curves shown in FIG. 1A are linear (straight lines).

High-Compliant balloons are made from relatively soft or flexiblepolymeric materials. Examples of these materials are thermoplasticpolymers, thermoplastic elastomers, polyethylene (high density, lowdensity, intermediate density, linear low density), various copolymersand blends of polyethylene, ionomers, polyesters, polyurethanes,polycarbonates, polyamides, polyvinyl chloride,acrylonitrile-butadiene-styrene copolymers, polyether-polyestercopolymers, and polyether-polyamide copolymers. A suitable copolymermaterial, polyolefin material is available from E. I. DuPont de Nemoursand Co. (Wilmington, Del.), under the trade name Surlyn® Ionomer.

Intermediate-Compliant balloons are made of polyethylene and nylonmaterials.

Non-Compliant balloons are made from relatively rigid or stiff polymericmaterials. These materials are thermoplastic polymers and thermosetpolymeric materials. Some examples of such materials are poly(ethyleneterephthalate), polyimide, thermoplastic polyimide, polyamides,polyesters, polycarbonates, polyphenylene sulfides, polypropylene andrigid polyurethanes. Non-Compliant balloons made from poly(ethyleneterephthalate) are commonly referred to as PET balloons.

The compliant characteristics of an angioplasty balloon affects how thephysician uses the balloon catheter. A Non-Compliant balloon, willincrease in diameter by a maximum of 1-15% of its nominal diameter inresponse to increasing the pressure to as much as twenty atmospheres.Sixteen atmospheres is safely below the burst pressure of such aNon-Compliant balloon. However, when inflated to its expanded diameter,a Non-Compliant balloon becomes very hard.

When a physician encounters a lesion that has become calcified and isvery hard and rigid he may select a Non-Compliant balloon, that willbecome very hard and function to crack the rigid calcified lesion.Non-Compliant balloons have the advantage over Compliant balloons inthat they can be used to dilate and crack hard lesions. Also if aNon-Compliant balloon is located in a vessel, across a restricted areaof the vessel, and an end or both ends extend into non restricted areasof the vessel, the pressure in the balloon can be increased in theballoon sufficient to dilate or crack the restricted area withoutrisking the possibility of damaging adjacent non restricted portions ofthe vessel. Non-Compliant balloons have the disadvantage that they arenot effective if the normal vessel size lies between the size range ofthe available Non-Compliant balloons. Another disadvantage ofNon-Compliant balloons is that if the lesion or restriction recoilsafter being dilated to its desired diameter, the Non-Compliant ballooncannot be used to dilate the lesion or restriction to a diameter greaterthan the previous dilation to thus overcome the recoil.

A High-Compliant balloon, will increase in diameter 15% to 40% inresponse to increasing the inflation pressure to a point safely belowits burst pressure. The advantage of a High-Compliant balloon over aNon-Compliant balloon is that fewer models of High-Compliant balloonsare required to fill a range of sizes. Non-Compliant balloons aretypically available in size increments of 0.25 mm while High-Compliantballoons typically have size increments of 0.50 mm. Also an off-sizedartery (i.e. 2.90 mm) will be difficult to dilate with a Non-Compliantballoon. Another advantage of a High-Compliant balloon over aNon-Compliant balloon is that if a restriction, after being dilated toits desired diameter, recoils when the balloon is deflated, theHigh-Compliant balloon can be re-inflated to a higher pressure thusdilating the restriction to a diameter greater than its desired diameterresulting in a satisfactory post recoil lumen diameter. This process canbe repeated until the restriction retains its desired diameter afterdeflation of the balloon. High-Compliant balloons also havedisadvantages, for example they can not be successfully used to dilate ahard lesion. Also if a High-Compliant balloon is located across arestriction and an end or both ends of the balloon extend into nonrestricted areas, when high pressure is applied to the balloon, thepressure may not be sufficient to crack or dilate the restrict area butwill dilate the non restricted area to diameters greater than theirnormal diameter. In this situation damage can be done to the nonrestricted portions of the vessel.

Compliance curves of angioplasty balloons, in their usable range arelinear, that is essentially a straight line. As a result a physicianschoice, in the past, has been to select a balloon having a linearcompliance curve that best meets his needs. Physicians often encountermedical situations where an angioplasty balloon having a nonlinearcompliance curve is called for but balloon catheters with the desiredcompliance curve have not been available. For example a physician mayhave a medical situation in which he desires a balloon that will duringthe initial inflation phase increase in diameter by 20% and then in thesecondary inflation phase become very rigid and hard with little furtherincrease in diameter. Another example would be the situation where twolesions are encountered, one that can be treated with a High-Compliantballoon and the other that requires a Non-Compliant balloon.

The prior art discloses balloon catheters and methods for making ballooncatheters in which the balloons have linear compliance curves. Referencemay be had to U.S. Pat. No. 4,456,000 and U.S. Pat. Re. Nos. 32,983 and33,561 for disclosures of methods for making balloon catheters havinglinear compliance curves.

In some situations a physician may desire a High-Compliant balloon thatcan initially expanded a significant amounts. If after the blood vesseladjacent to the restriction has been dilated to its natural size or atmost 10% larger than its natural size, and the lesion has not yieldedcompletely, it is not desirable that the balloon size be furtherincreased due to the high rate of restenosis and dissection. In asituation such as this the physician may, after the restriction has notyielded sufficiently with the High-Compliant balloon, desire to removethe High-Compliant balloon and replace it with a Non-Compliant balloon.The Non-Compliant balloon that would be selected in this situation wouldhave a nominal diameter approximately equal to the natural diameter ofthe open blood vessel, and its desired function would be to tightlycompress the lesion into the wall of the blood vessel. It is desirablein this situation that the inflated balloon becomes very hard and rigidbut not expand to a diameter that is greater than the natural diameterof the blood vessel. To accomplish this with currently available ballooncatheters the initial High-Compliant balloon must be removed andreplaced with a Non-Compliant balloon. This has the disadvantage thatthe patient is exposed to the trauma of removing and replacing a ballooncatheter, the procedure time is lengthened and there is the expense oftwo balloon catheters. These disadvantages can be avoided by use of aballoon catheter that has a nonlinear or hybrid compliance curve.

A hybrid compliant balloon is a balloon that has a nonlinear or hybridcompliance curve. A benefit of a hybrid compliant balloon is thatadvantages of both the Compliant and Non-Compliant balloons can beobtained in a single catheter that can be sized to the artery by varyingthe inflation pressure.

FIG. 2A is a graph in which the balloon diameter, in millimeters, isplotted along the Y-axis and the pressure in atmospheres is plottedalong the X-axis. FIG. 2A, shows the compliance curves for twoparticular balloon catheters. In this figure the compliance curve havingthe greatest inclination is labeled High-Compliant and the compliancecurve having the lesser inclination is labeled Non-Compliant. A hybridballoon combines these two compliance curves and the result is as shownby the full line compliance curve in FIG. 2A. The full line compliancecurve of FIG. 2A is a non linear compliance curve which is a hybrid ofthe compliance curves of a compliant and a non compliant balloon.

A balloon catheter having a hybrid compliant curve is disclosed andclaimed in commonly owned and pending application Ser. No. 07/927,062,filed Aug. 6, 1992, the disclosure of which is incorporated herein byreference.

In the above referred to copending application a double layered balloonis used to attain the hybrid compliance curve. The term "profile" asused with reference to balloon catheters refers to the diameter of thedilating element or balloon when it is un-inflated. Usually anun-inflated balloon will be folded down to minimize its diameter orprofile. A small profile enables the balloon catheter to be manipulatedthrough restrictions and around sharp curves. Unless the dilatingelement of a balloon catheter can reach and extend through tightlyclosed lesion it cannot perform its task of dilating the lesions orrestriction. A double layered balloon will have a larger profile than acomparable single layered balloon. Although a double layered balloonincreases the profile of a balloon catheter, having a hybrid compliancecurve outweigh the disadvantages of the increased profile. However asingle layered balloon catheter that has a hybrid compliance curve iseven more advantageous.

The stiffness or flexibility of the un-inflated dilating element orballoon portion of a balloon catheter is another factor contributing tothe maneuverability of a balloon catheter. The more flexible the balloonsection is the easier it is to manipulate it through sharp turns andsmall radius. A single layered dilating element is much moremaneuverable than a double layered dilating element.

It is a primary objective of the present invention to provide alow-profile balloon catheter having a hybrid compliance curve that has aparticular use in a medical procedure.

Another objective of the present invention is to provide a method formanufacturing a single layered balloon catheter having a hybridcompliance curve.

Another objective of the present invention is to provide a single layerballoon catheter, having a high compliance curve in the lower inflationrange and a low compliance curve in the higher inflation range, thattogether combine to provide a hybrid compliance curve.

Still another objective of the present invention is to provide a ballooncatheter that has a high compliance curve in the lower inflation rangeand a low compliance curve in the higher inflation range and isreversible through multiple inflations within the lower inflation range.

SUMMARY OF THE INVENTION

To achieve these and other objectives, the present invention provides anew and unique single layer balloon catheter and method of producing theunique single layer balloon catheter that causes the balloon to have ahybrid compliance curve.

A preferred embodiment of the present invention includes a singlelayered balloon, that is molded from a polymeric material to a specificnominal diameter, the balloon is then shrunk to a diameter that is lessthan the original diameter. The resulting shrunk balloon has a highcompliance curve when inflated and stretched from its shrunk diameter toa diameter that is approximately equal to its original specificdiameter. The balloon catheter can be subjected to multipleinflation-deflation cycles and it will maintain its high compliancecurve provided it is not inflated sufficiently to extend into the noncompliant portion of its hybrid compliance curve. When the ballooncatheter has been expanded to a diameter that is approximately equal toits original diameter, then further inflation of the balloon will resultin its expansion following a non-compliant curve. As a result such aballoon catheter has a hybrid compliance curve. However, once theballoon has reached its non compliant phase its compliantcharacteristics have been lost. If such a balloon is deflated and thenreinflated it will follow a non compliant compliance curve during itsentire expansion.

An important advantage of the present invention is that a physician canselect a low profile balloon catheter that will perform certain desiredfunctions when located in the vascular system and inflated to particularpressures that cannot be performed with currently available ballooncatheters.

It is another advantage of the present invention that a physician willhave a greater variety of balloon catheters to choose from that willhave the capability to perform new functions or combinations offunctions that formally required multiple balloon catheters.

These and other objects and advantages of the present invention will nodoubt become apparent to those skilled in the art after having read thefollowing detailed description of the preferred embodiment which arecontained in and illustrated by the various drawing figures.

BRIEF DESCRIPTION OF DRAWING

FIG. 1A is a graph showing compliance curves for several dilatingcatheters with the diameter of the balloon, measured in millimeters, asthe ordinate and inflating pressure, measured in atmospheres, as theabscissa.

FIG. 2A is a graph, with balloon Diameter measured in millimeters as theordinate and inflating pressure measured in atmospheres as the abscissa,on which is shown the compliance curve for two different balloons andalso the combined hybrid compliance curve.

FIG. 3 shows a dilation catheter in cross-section including a balloonhaving a hybrid compliance curve.

FIG. 4 shows the machine used to form the balloon used to produce aballoon catheter in accordance with this invention.

FIG. 5 shows the balloon mold used in the machine shown in FIG. 4.

FIG. 6 is an enlarged view of the lower mold gripper that includesdetails not visible in FIG. 4.

FIG. 7 is a graph, with balloon Diameter measured in millimeters as theordinate and inflating pressure measured in atmospheres as the abscissa,in which compliance curves of a balloon made in accordance with thisinvention is plotted.

FIG. 8 is a graph, of a balloon made in compliance with this inventionin which the balloon diameter measured in millimeters is plotted as theordinate and inflating pressure measured in atmospheres as the abscissa.The balloon used for this graph was inflated and its diameter measuredat 3, 6 and 9 atmospheres and then the balloon was deflated and theprocess repeated several times. The fourth inflation was continued to 24atmospheres.

FIG. 9 is a graph, of a balloon made in compliance with this inventionin which the balloon diameter measured in millimeters is plotted as theordinate and inflating pressure measured in atmospheres as the abscissa.The balloon used for this graph was inflated and its diameter measuredat 3, 6, 9 and 12 atmospheres and then the balloon was deflated and theprocess repeated several times. The fourth inflation was continued to 23atmospheres.

FIG. 10 is a graph, with balloon diameter measured in millimeters as theordinate and inflating pressure measured in atmospheres as the abscissa,in which the compliance curves of a series of balloons manufactured inaccordance with this invention have been plotted.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention will be illustrated and described as anover-the-wire balloon catheter for use in angioplasty of the typeillustrated in FIG. 3. However it should be understood that the presentinvention can be applied to fixed-wire catheters including shortenedguide wire lumens or to non-over-the-wire balloon catheters. Furthermorethis invention can be used in balloon catheters intended for use in anyand all vascular systems or cavities of the body.

Referring to FIG. 3 which shows the distal end of a balloon catheter140. The balloon catheter 140 illustrated in FIG. 3 is an example of thetype of catheter that the subject invention can be applied to but itshould be understood that the invention can be applied to any type ofballoon catheter. Furthermore, although several embodiments of ballooncatheters having hybrid compliant curves will be disclosed herein theirvisual appearance is the same, in fact their visual appearance is nodifferent than current balloon catheters. The balloon 2 is constructedof material such as poly(ethylene terephthalate) which is referred to asa PET balloon. Conventional balloon catheters made from PET have a lowcompliant compliance curve. The catheter is made up of an elongatedouter plastic tube 4 having a distal end 6. The plastic tube 4 ispreferably made of a flexible material such as a high densitypolyethylene. The elongated outer plastic tube 4 has a lumen 8 thatfunctions as the inflation lumen and extends its entire length. There isa recessed area 10 at the distal end 6 into which the proximal end 12 ofthe balloon 2 is secured by bonding. The balloon 2 is molded to adesired shape, size and wall thickness. The shape of balloon 2 isgenerally cylindrical with reduced portions at each end. The proximalend 12 of balloon 2 is bonded to the recessed area 10 formed in thedistal end 6 of the elongated outer plastic tube 4.

An elongated inner tube 16 is concentric with and within the elongatedouter plastic tube 4. The inflation lumen 8 is defined by the innersurface of elongated outer plastic tube 4 and the outer surface ofelongated inner tube 16. The distal end 18 of elongated inner tube 16extends distally of the distal end 6 of elongated outer tube 4. Thedistal end 14 of balloon 2 is adjacent to the distal end 18 of theelongated inner tube 16 and is secured thereto by bonding. The elongatedinner tube 16 is hollow and thus forms a guide wire lumen 20. Since theun-inflated diameter of an angioplasty balloon is generally greater thanthe diameter of the outer plastic tube 4 it is the usual practice tofold down the balloon and wrap it in the folded condition such that itwill maintain a low-profile during un-inflated use. Balloon cathetershaving a low profile are easier to manipulate through the patientsvascular system and is particularly beneficial when passing the balloonthrough a tightly closed lesion.

The process for making a balloon for a balloon catheter than will havethe hybrid compliance curve of this invention includes three majorprocess; extruding of a tube, blow molding the balloon and annealing theballoon. Each of these major process include a number of process steps.

Polyethylene terephalate (PET) homopolymers and copolyesters can be usedas the extrusion material. The copolyesters can be synthesized fromethylene glycol and a mixture of dimethyl terephthalate and dimethylisophthalate by a two-step ester interchange process. The copolyesterscan be branched via trifunctional compounds. The composition of thethird component is in the range of 0.001%-40%. The following, Table 1,provides physical properties of the homopolyester and copolyesters usedin the extrusion process.

                  TABLE I                                                         ______________________________________                                        Physical properties of PET Homopolymer and Copolymers                                              COPOLYESTERS                                             Properties   HOMOPOLYMER   I      II   III                                    ______________________________________                                        Ratio of terephthalate                                                                     100/0         97/3   92/8 87/13                                  to isophthalate                                                               Intrinsic viscosity                                                                        0.72          0.80   0.80 0.68                                   Melting T °C.                                                                       250           248    230  221                                    ______________________________________                                    

The polyethylene terephthalate homopolymers and copolymers resins aredried by a hot air dryer using -40° Fahrenheit dew point air in a plenumstyle hopper. The polymer moisture content is controlled within a rangeof 1 to 90 parts per million by varying the drying temperature and time.

The extruding machine has a length to diameter ratio of 20:1 and has aminimum of three temperature control zones and additional heater zonesfor the adapter, head and die. The temperature controllers are of theproportioning type that can maintain temperatures within precise limitsto thereby provide a homogeneous melt. The materials from which thebarrel and screw components of the extrusion machine are conventionalbimetallic, surface hardened and chrome plated. A conventional screwwith a 3:1 compression ratio and a relatively constant transition fromfeed to metering zone has been successfully used in the process. Howevera costumed designed screw has been designed to improve the meltuniformity and mixing without excessive shear or polymer degradation.The extrusion machines breaker plate, adapter, head and tooling havebeen designed such that they provide gradual transitions, rounded edgesand a minimum of obstructions. Screen packs of 60-80-60and 60-1400-60meshes have been found to generate adequate back pressure. The die andtip drawdown ratios are maintained between 2:1 and 3:1 and the die landlengths are 10 to 20 times the part wall thickness. Sizing of the tubeis accomplished by free extrusion while maintaining constant nitrogenpressure inside the tubing while being quenched in a conventional waterbath at ambient temperatures. Specific extrusion parameters that wereused to extrude tubes from which balloons having hybrid compliancecurves in accordance with this invention are listed in Table II.

                                      TABLE II                                    __________________________________________________________________________    Extrusion Conditions of Polyesters                                            Polymer                                                                            Temperature °C.  Melting                                          Types                                                                              Zone 1                                                                            Zone 2                                                                            Zone 3                                                                            Zone 4                                                                            Zone 5                                                                            Zone 6                                                                            Pressure                                                                           Temperature                                 __________________________________________________________________________    PET  275 272 267 259 258 275 2900 psi                                                                           272° C.                              PET  244 254 260 261 259 260 3017 psi                                                                           265° C.                              Co I 248 249 249 249 248 265 4100 psi                                                                           258° C.                              Co II                                                                              244 243 243 244 244 257 6600 psi                                                                           254° C.                              Co III                                                                             229 232 233 233 227 230 4800 psi                                                                           242° C.                              __________________________________________________________________________

The process for molding or blowing a dilation catheter balloon that canbe used to incorporate this invention will be discussed with referenceto FIG. 4. This discussion will describe the process for producing aspecific dilation catheter balloon, thus the material from which it ismade, its dimensions and specific temperatures and time periods used inthe process will be included in the example. However it should beunderstood that this explanation relates to a specific balloon and theinvention is not limited to this specific example. It should also benoted that the specific balloon discussed with reference to FIG. 4 couldbe used in the construction of a balloon catheter of the typeillustrated in FIG. 3 or other types of balloon catheter.

This invention has been applied to balloons made from polyethyleneterephalate (PET) homopolymers and copolyesters. The copolyesters usedin these balloons can be synthesized from ethylene glycol and a mixtureof dimethyl terephthalate and dimethyl isophthalate. Copolyesters can bebranched via trifunctional compounds. The composition of the thirdcompound is the range of 0.001%-40%. The Intrinsic viscosity (IV) of thePET homopolymer material was 0.72 and for the copolyesters 0.68 and0.80.

The process begins with a length of extruded tube, about 20 centimeterslong, made from the balloon material. Balloons formed from the extrudedtubes as they exist after extrusion have been successfully processed toincorporate this invention of a balloon catheter having a hybridcompliance curve. However the tube as it is received from the extrusionprocess can be pre stressed or stretched prior to subjecting it to theblow molding process.

When it is desired to stretch the extruded tube, the starting tube isstretched to 1.5-3.0 times its initial length while submerged in waterat a temperature of 90° Centigrade. The preferred stretching ratio is2.25. This stretching of the starting tube contributes further to theorientation of the molecules along the length of the tube. It ispreferred to mold the balloon soon after the tube has been extruded, toinsure that conditions of the tube, such as moisture content remainacceptable.

The apparatus or blowing machine 100 shown in FIG. 4 receives theballoon mold 102 and supports and manipulates it through the balloonforming process. The blowing machine 100 is supported by an arm 101 thatcan be reciprocated vertically and oscillated, as indicated by thearrows. The blowing machine 100 includes a lower mold gripper 144 thathas a channel 146 and an upper mold gripper 148 that has a channel 150.The upper and lower mold grippers 148 and 144 are secured to a verticalshaft 152 that is secured at its upper end to a locking device 154 whichis mounded on a frame member 156. This movement enables the mold 102 tobe indexed to a location over a hot water tank 132 and then a quenchingtank 134 and submerged in these tanks for specific time periods. Itshould be noted that although the tanks 132 and 134 are filled withwater at the appropriate temperature this operation can also beperformed with air at the appropriate temperature. The movement impartedto arm 101 can be controlled by an computer controlled device, which isnot considered to be a part of this invention.

The stretched tube 120 is placed in the mold 102, which as can be bestseen in FIG. 5 is a three part mold. Mold 102 includes an upper moldportion 110, a lower mold portion 112 and a central mold portion 114.The upper mold portion 110 has an upper diverging section 116 and acylindrical section 106. The lower mold portion 112 has a lowerdiverging section 118 and a cylindrical section 104. The diameter of thecylindrical sections 104 and 106 being slightly larger than the diameterof the stretched tube 120 that is to be placed in the mold. The upperand lower mold portions 110 and 112 are counterbored at 117 and 119respectively. The central mold portion 114 has a cylindrical section 115that has a diameter corresponding to the nominal diameter of the balloonto be produced by the mold. The central mold portion 114 has a reducedouter diameter area at both ends that are received in counterbores 117and 119 to thereby connect the three parts of the mold together.

Alternatively, both end portions of the stretched tube 120 are thenstretched further, leaving a central section 142 (see FIG. 5) that hasnot been subjected to this further stretching. The length of the centralsection 142 is slightly less then the distance L, identified in FIG. 5,which extends from the juncture of the diverging section 116 with thecylindrical section 106 to the juncture of the diverging section 118with the cylindrical section 104. This further stretching of the tube120 has caused the central section 142 to have a wall thickness that isgreater than the end portions of the tube. As shall be discussed later,during the balloon blowing step an upward force is exerted on the tube120 causing it to be stretched in the longitudinal direction.

The further stretched tube 120 is placed in the three part mold 102 suchthat it fits snugly in the upper 110 and lower 112 tube portions andextends along the axis of the central balloon portion 114. The tube 120is placed in the mold such that the bottom edge of the central section142 is at the intersection of the lower diverging portion 118 and thecylindrical section 104 of the mold. Since the length of the centralsection 142 is slightly shorter than the length L, the upper edge of thecentral section 142 will be below the intersection of the upperdiverging portion 116 with the cylindrical section 106.

The mold is then secured to the molding machine 100 by placing the upperend of the mold in the upper mold gripper 148 with the tube lying in thechannel 150 and the lower end of the mold in the lower mold gripper withthe tube 120 lying in the channel 146. As can be best seen in FIG. 6,the lower mold gripper 144 has a clamping device 108 that includes apoint 109. When the clamping device 108 is closed point 109 presses thetube 120 against the surface of channel 146 to prevent gas from leakingout this end of the tube 120.

A connector 122, supported by the frame 156, is provided for connectingthe upper free end of tube 120 to a source of pressurized gas 124 suchas Nitrogen. The pressure source 124 can apply a force up to 300 psiinternally of tube 120. However a force of 200-270 psi is used mostoften. The tube 120 extends through and is griped by lifting block 125which is located between the upper end of mold 102 and the connector122. The section of tube 120 that extends from lifting block 125 toconnector 122 is provided with a slight amount of slack for a purpose tobe discussed. The lifting block 125 functions to exert an upward forceon the tube 120 in such a way that the internal passage through tube 120remains open and is not collapsed. The upward force can be applied tothe lifting block, for example, by connecting a wire 126 to the liftingblock 125, extending the wire over a pulley 128, and connecting a weight130 to the free end of wire 126. A 50 gram weight has been used in thepreferred embodiment of the blowing machine, however the actual amountof this weight would be a factor of the size of the balloon and startingtube. The pulley 128 is mounted for rotation on a post 158 that extendsupwardly from the frame 156.

The arm 101 then indexes the blowing machine 100 horizontally so that itoverlies the hot water tank 132 and then dips the mold 102 into the tank132 which contains water at about 95° Centigrade. In the preferredprocess the bottom half of the mold is first submerged for a 15 secondperiod and then the entire mold is submerged for an additional 40seconds. Pressure is applied to the interior of tube 120 throughpressure source 124 during the time period that the mold is submerged inthe hot water. A pressure in the range of 100-300 psi, with a preferredrange of 200-270 psi is applied. The internal pressure causes the tube120 to expand radially in the central balloon portion 114 of the mold.The diameter of the tube 120 expands in a range of 3-10 times itsoriginal diameter however the preferred ratio is in the range of 6-9times its original diameter. Although most of the expansion during thisstep of the process occurs in the radial direction some additionallongitudinal stretching also occurs. This longitudinal stretching is afactor of the upward force applied to the tube by the weight 130. Duringthis phase of the process the lifting block 125 visually moves upwardrelative to the top end of mold 102. This upward movement pulls theupper end of tube 120 upwardly out of the upper end of mold 102 andcauses the upper edge of the central section 142 to move upwardly suchthat it is at the intersection of the upper tube portion 110 and theupper diverging portion of the mold. As a result the expanded balloon isformed from the central section 142 which had a greater wall thicknessthan the end portions of tube 120 prior to blowing the balloon. Afterthe balloon has been blown the wall thickness of the end portions andthe balloon portion has been reduced as a result of the expansion thathas occurred. The upward movement can be accommodated by the slack thatwas left in the section of tube 120 that extends from lifting block 125and the connector 122.

The entire mold is then removed from the 95° Centigrade water bath,indexed horizontally until it overlies quenching tank 134. The entiremold 102 is then dipped into and quench, in room temperature water, forabout 10 seconds.

The blowing machine 100 is then removed from the room temperaturequench, the mold 102 is released from the blowing machine 100 and openedup. The formed balloon is then released and removed from the mold 102.The free ends of the balloon are then trimmed and the balloon catheteris assembled. An assembled over-the-wire type balloon catheter 140, isillustrated in FIG. 3.

The final process that the assembled balloon catheter must undergo isthe annealing technique. The entire balloon catheter is submerged inwater or air at a temperature in the range of 25°-100° Centigrade for3-180 minutes. The preferred temperature range being 65°-80° Centigrade.It should be understood that the temperature and time required in thisannealing process depends upon the size of the balloon that is beingprocessed. This annealing process causes the length and the diameter ofthe balloon to decrease and the wall thickness to increase. After theballoon catheter has been removed from the annealing process it issterilized under ethylene oxide at 47° Centigrade.

FIG. 7 is a graph, with pressure in atmospheres plotted on the X axisand Diameter in millimeters plotted on the Y axis, of a balloon that hasbeen manufactured in accordance with this invention. The balloon usedfor this graph had an original nominal diameter before being shrunk of2.75 millimeters. The curve formed by connecting the circles is thecompliance curve for the shrunk balloon when it is originally inflated.This compliance curve, at pressures less than 12 atmospheres, isrelatively steep and has the characteristics of a high compliantcompliance curve. The section of this curve above 12 atmospheres has avery shallow pitch which is characteristic of a non compliant compliancecurve. The curve in FIG. 7, that is formed by connecting the solid dots,is the compliance curve of the same balloon after it has been deflatedfrom a pressure above 12 atmospheres and then reinflated. This secondcompliance curve has the same pitch throughout its entire extent thatthe earlier discussed curve had above 12 atmospheres. Thus it isapparent that the balloon having been once inflated above 12 atmosphereshas lost its compliant characteristics, and when reinflated it follows anon compliant compliance curve rather than a hybrid compliance curve

Referring now to the graph shown in FIG. 8, in which pressure inatmospheres is plotted on the X axis and balloon diameter in millimetersis plotted on the Y axis. The compliance curve of a balloon that hasbeen manufactured in accordance with this invention has been plotted onthis graph. The balloon plotted on this graph, like the balloon used forthe FIG. 7 had a nominal diameter prior to being shrunk of 2.75millimeters. In collecting the data for this graph the pressure in theballoon was increased and the diameter measured at 3, 6 and 9atmospheres. The balloon was then deflated and the process of increasingthe pressure and recording the diameter at 3, 6 and 9 atmospheres wasrepeated. The balloon was then deflated again and the process ofinflating and recording the diameter was repeated a third time. Oncemore the balloon was deflated and the process of inflating and recordingthe diameter was began a fourth time. However on the fourth cycle thepressure was increased and the diameter recorded up to 24 atmospheres.The balloon displayed the characteristics of a compliant balloon in eachof the four inflations up to 9 atmospheres and then followed a noncompliant compliance curve when in the fourth inflation the pressure wasincreased above 12 atmospheres.

Referring now to the graph shown in FIG. 9, in which pressure inatmospheres is plotted on the X axis and balloon diameter in millimetersis plotted on the Y axis. The compliance curve of a balloon that hasbeen manufactured in accordance with this invention has been plotted onthis graph. The balloon plotted on this graph, like the balloon used forthe FIGS. 7 and 8 had a nominal diameter prior to being shrunk of 2.75millimeters. In collecting the data for this graph the pressure in theballoon was increased and the diameter measured at 3, 6, 9 and 12atmosphere. The balloon was then deflated and the process of increasingthe pressure and recording the diameter at 3, 6, 9 and 12 atmosphere wasrepeated. The balloon was then deflated again and the process ofinflating and recording the diameter was repeated a third time. Oncemore the balloon was deflated and the process of inflating and recordingthe diameter was began a fourth time. However on the fourth cycle thepressure was increased and the diameter recorded up to 23 atmospheres.The balloon displayed the characteristics of a compliant balloon in eachof the four inflations up to 12 atmospheres and then followed a noncompliant compliance curve when in the fourth inflation the pressure wasincreased to 23 atmospheres.

The graphs of FIGS. 8 and 9 illustrate that balloon manufactured inaccordance this invention have reversible compliant characteristicsprovided they are not inflated beyond the nominal diameter that they hadprior to shrinking.

Referring now to FIG. 10 which is a graph, with pressure in atmospheresplotted on the X axis and diameter in millimeters plotted on the Y axis.The compliance curves of a series of balloons, having pre shrunk nominaldiameters from 2.75 millimeters to 4.25 millimeters, that has beenmanufactured in accordance with this invention are plotted on thisgraph. An individual symbol such as a solid square or an outline of atriangle have been used to identify the data points for each curve inthe series. Although the extruded tubes from which these individualballoons were blown and the resulting balloon wall thicknesses were notidentical, the resulting compliance curves are all similar and all arenonlinear.

Although the present invention has been described in terms of specificembodiments, it is anticipated that alterations and modificationsthereof will no doubt become apparent to those skilled in the art. It istherefore intended that the following claims be interpreted as coveringall such alterations and modifications as fall within the true spiritand scope of the invention.

We claim:
 1. A catheter comprising:an elongated catheter tube having aproximal and a distal end, a single layered dilating element secured tothe distal end of said elongated catheter tube, said single layereddilating element having a hybrid compliance curve that includescompliant and non compliant portions, said elongated catheter tubehaving an inflation lumen extending from its proximal to its distal end,the distal end of said inflation lumen communicating with the interiorof said single layered dilating element, inflation means incommunication with the proximal end of said inflation lumen for meteringinflation fluid into the inflation lumen and the interior of said singlelayered dilating element to cause the dilating element to expand in thediametric direction in conformance with its hybrid compliance curve. 2.The invention as set forth in claim 1 wherein the hybrid compliancecurve includes a relatively steep slope section corresponding to thecompliant portion and a relatively shallow slope section correspondingto the non compliant portion.
 3. The invention as set forth in claim 2wherein the relatively steep slope section corresponds to the initialinflation phase of the catheter and the relatively shallow slope sectioncorresponds to the secondary inflation phase of the catheter.
 4. Theinvention as set forth in claim 1 wherein the compliant characteristicsof the single layered dilation element is reversible through multipleinflation-deflation cycles provided the single layered dilation elementis not inflated sufficiently to extend into the non compliant portion ofits hybrid compliance curve.
 5. The invention as set forth in claim 1wherein the single layered dilating element is made of a copolyestermaterial.
 6. The invention as set forth in claim 1 wherein the singlelayered dilating element is made of a homopolymer material.
 7. Theinvention as set forth in claim 1 wherein the single layered dilatingelement is made of a polyethylene terephalate homopolymer material. 8.The invention as set forth in claim 1 wherein the single layereddilating element is made of a polymer having an intrinsic viscosity thatis less than 1.00.
 9. A catheter including an inflatable balloon portionhaving a hybrid compliance curve that includes a compliant and a noncompliant portion:a balloon molded from non compliant plastic materialhaving an original non-compliant diameter and then shrunk to a seconddiameter that is smaller than said original non-compliant diameter,means for inflating the balloon, said balloon following a compliantcompliance curve during initial inflation until it has been stretched toa diameter that is substantially equal to the original non-compliantdiameter and upon further inflation it follows a non compliantcompliance curve.
 10. The invention as set forth in claim 9 wherein theballoon is made of a copolyester material.
 11. The invention as setforth in claim 9 wherein the balloon is made of a homopolymer material.12. The invention as set forth in claim 9 wherein the balloon is made ofa polyethylene terephalate homopolymer material.
 13. The invention asset forth in claim 9 wherein the balloon is made of a polymer having anintrinsic viscosity that is less than 1.00.
 14. The invention as setforth in claim 12 wherein the balloon is made of a polymer having anintrinsic viscosity that is less than 1.00.
 15. The invention as setforth in claim 9 wherein the compliant characteristics of the balloonare reversible through multiple inflation-deflation cycles provided theballoon is not expanded larger than its original non-compliant diameter.16. A catheter including an inflatable balloon portion having a hybridcompliance curve that is advantageous in particular medical applicationscomprising:a balloon, said balloon having been molded to a specificdiameter and being formed of a plastic material, said molded balloonhaving been shrunk to a diameter that is less than its original specificdiameter, means for inflating the balloon such that upon initialapplication of an inflation media to the balloon portion of thecatheter, the balloon will be inflated and will follow a compliantcompliance curve, after which and upon the application of increasedpressure through the inflation media the balloon will follow a noncompliant compliance curve.
 17. The invention as set forth in claim 16wherein the balloon is made of a copolyester material.
 18. The inventionas set forth in claim 16 wherein the balloon is made of a homopolymermaterial.
 19. The invention as set forth in claim 16 wherein the balloonis made of a polyethylene terephalate homopolymer material.
 20. Theinvention as set forth in claim 16 wherein the balloon is made of apolymer having an intrinsic viscosity that is less than 1.00.
 21. Theinvention as set forth in claim 19 wherein the balloon is made of apolymer having an intrinsic viscosity that is less than 1.00.